As a fixing device to be mounted on a copying machine or a printer, there is a device having an endless belt, a ceramic heater which contacts an inner surface of the endless belt, and a pressure roller which forms a fixing nip portion together with the ceramic heater via the endless belt.
When small size sheets are continuously printed by the image forming apparatus that mounts thereon the fixing device, there occurs a phenomenon (non-sheet-passing portion temperature rise) that a temperature of a region where the sheets do not pass in a lengthwise direction of the fixing nip portion, is gradually rising.
When the temperature at the non-sheet-passing portion becomes too high, it may cause damages to respective portions in the device, or when printing is performed on large size sheets while the non-sheet-passing portion temperature rise is occurring, toner may be subjected to a high-temperature offset in a region corresponding to the non-sheet-passing portion of the small size sheets.
As a method for reducing the non-sheet-passing portion temperature rise, as discussed in PTL 1, there is devised a method for providing a first heat-generation line that has a resistance distribution in which a resistance value per unit length in a lengthwise direction gradually decreases from the central portion towards both end portions in a lengthwise direction of a heater, and a second heat-generation line that has a resistance distribution in which a resistance value per unit length in the lengthwise direction gradually increases from the central portion towards the both end portions in the lengthwise direction of the heater, and independently controlling the states of power supply, and shutoff of the respective heat-generation lines.